Currently in RF applications there are two types of switches that can be used to perform switching functions. The most popular and commercially available is the semiconductor device. This includes such devices as PIN diodes, field effect transistors (FETs) and heterojunction bipolar transistors (HBTs). They can be very lossy, expensive to fabricate, and complicate integration with other ICs. The other switch, more recently developed and commercially available to a limited extent, is the RF MEMS switch.
Typical RF MEMS switches are developed with standard IC processing which make fabrication and integration low cost and simple. However typical RF MEMS switches have cantilevers/bridges constructed from a single metal layer or a combination of metal layers.
A microelectromechanical system (MEMS) is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are formed using known integrated circuit fabrication techniques, while the mechanical elements are fabricated using lithographic techniques that selectively micromachine portions of a substrate. Additional layers are often added to the substrate and then micromachined until the MEMS device is in a desired configuration. MEMS devices include actuators, sensors, switches, accelerometers, and modulators.
MEMS switches have intrinsic advantages over conventional solid-state counterparts such as field-effect transistor switches. The advantages include low insertion loss and excellent isolation. However, MEMS switches are generally much slower than solid-state switches. This speed limitation precludes applying MEMS switches in certain technologies, such as wireless communications, where sub-microsecond switching is required.
One type of MEMS switch includes a suspended connecting member, or beam, that is electrostatically deflected by energizing an actuation electrode. The deflected beam engages one or more electrical contacts to establish an electrical connection between isolated contacts. A beam anchored at one end while suspended over a contact at the other end is called a cantilevered beam. A beam anchored at opposite ends and suspended over one or more electrical contacts is called a bridge beam.
There are two types of MEMS switches: metal-to-metal contact and capacitive. Metal-to-metal contact switches consist of a metal transmission line and a metal bridge/cantilever that are separated by an air gap. This type of switch requires a DC voltage to actuate but suffers from the metal contacts wearing down and welding after prolonged use, thus causing the switch to fail. The other type, capacitive MEMS switches, employ a thin insulator and air gap between the transmission line and the bridge/cantilever to prevent the two metal structures from touching in an effort to prevent the metal connects from welding together. This type of switch requires a low voltage peak-to-peak sine wave voltage to actuate. The low frequency peak-to-peak voltage waveform is needed to prevent charge trapping within the insulator film and seriously degrades the performance of the switch and complicates the biasing structure of the overall system.
However, the greatest disadvantage of the two types (contact and capacitive) of MEMS switches is that they utilize metal bridge/cantilever structures that are very unreliable due to severe sagging and eventual failure during prolonged operations. This drastically reduces the reliability of the switches. Reliability is the single most important issue now prohibiting metal-based RF MEMS switches from being implemented in a wide range of commercial applications, as well as applications for military and space.